Joining two cylinder at 45 degree angle
To make a pipe bend the quickest way is to use the Bridge tool: Select both the 2 rims vertices and right click (context menu) &gt; LoopTools &gt; Bridge (you may need to activate the LoopTools addon before), or in Edge Select mode, CtrlE &gt; Bridge Edge Loops. Then in the Operator box, set the bend as you want.

If you want to create a pipe bend but want to keep a sharp edge, the new (2.8) Mirror modifier allows you to do it easily: Create your cylinder, create an empty, give your cylinder a Mirror modifier with the empty as Mirror Object, choose (in my case) Axis &gt; X, Bisect &gt; X, Flip &gt; X, move and rotate the empty until the cylinder and its mirrored merge the way you want (keep pressing Ctrl during the rotation if you want it to be 10° by 10°).

The Shear tool also allows you to create sharp bends but it's not convenient if you want precise angles or if your object is not perpendicular to the axis.